Multi-branched or star-shaped polymers were described by Zelinski in U.S. Pat. No. 3,280,084, in which polybutadiene anion (generated by addition of butyllithium to butadiene) was coupled with 0.02 to 1 part by weight of divinylbenzene to 100 parts of monomer. The resultant star polymer had a polydivinylbenzene core and several identical arms of polybutadiene radiating from the core. The arms could also be either random or block copolymers of styrene and butadiene, wherein the diene is the major component. The synthesis of star polymers with arms of different molecular weights was disclosed by Kitchen et al, U.S. Pat. No. 3,639,517. Fetters and Bi, U.S. Pat. No. 3,985,830, detailed the preparation of star polymers with a nucleus of more than one molecule of divinylbenzene, and more than three arms. These polymers were formed by addition of living homopolymers and copolymers of conjugated diene monomers and block copolymers of conjugated diene monomers and monovinylaromatic monomers to divinylbenzene. Unfunctionalized star polymers have been employed as viscosity index (V.I.) improvers for lube oil compositions (see for example, U.S. Pat. Nos. 5,310,490 and 5,310,814).
Star polymers have also been prepared which contain functionality, such as hydroxy groups, at the ends of the arms. For instance, European Patent Application 0632075 is directed to the preparation of star molecules with hydroxyl groups at the terminus of each living arm of the star. However, these functionalization reactions are often inefficient, due to the formation of physical gelation phenomena that produce severe mixing problems (see L. Weber, Makromol. Chem., Macromol. Symp., 3, 317 (1986) and U.S. Pat. Nos. 5,393,843 and 5,478,899). These polymers were prepared by addition of ethylene oxide to the living anions at the ends of the arms of the star polymer. These functionalized star polymers are stated to be useful in making adhesives, sealants, coatings, films and fibers.
While these and other techniques for the synthesis of star polymers can be useful, they can also suffer various drawbacks. For example, these and other prior techniques can exhibit poor linking efficiencies of the living polymer anions. This in turn can result in lower yields and non-uniform chemical properties of the star polymers.